1. Field of the Invention
This invention relates to apparatus for modulating a carrier with information to be transmitted over a relatively low-bandwidth transmission medium in a manner to permit its detection at a receiver and in particular to such apparatus adapted to transmit data indicative of the width of a pacemaker stimulating pulse to a remote location, whereby the pulse width may be accurately determined to provide an indication of the pacemaker's energy source, e.g. a battery.
2. Description of the Prior Art
Heart pacemakers such as that described in U.S. Pat. No. 3,057,356, issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. A heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time, to provide cardiac stimulation at relatively low power levels by utilizing a small, completely implanted, transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. The electrical stimulation by this pacemaker is provided at a relatively fixed rate.
Such cardiac pacers of the implantable variety have found wire acceptance for patients suffering from complete heart block. As a result, the use of these pacers has increased the life expectancy of those patients with implants, from a 50% probability of one (1) year to nearly the life expectancy of physically-comparable humans not suffering from the same heart disorder.
Typically, such cardiac pacers are encapsulated in a substance substantially inert to body fluids, and are implanted within the patient's body by a surgical procedure wherein an incision is made in the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region, and a pacemaker is implanted therein. Due to the inconvenience, expense and relative risk to the patient's health, it is highly desired to extend the life of the power source or battery, whereby the number of such surgical procedures is limited. The resultant problem for the attendant doctor is to determine when the batteries should be replaced, keeping in mind the relative risk or probability of premature pacer failure due to battery depletion.
A number of solutions to this problem have been proposed, one being replacement at predetermined intervals, thus accepting an empirically determined risk or failure of the pacemaker batteries. Another proposed solution is to establish pacer "clinics" where photographic analysis techniques are used to detect imminent failure. These solutions are not entirely satisfactory for detecting, simply and positively, a degradation of pacer system performance. The risk of undetected premature failure associated with periodic replacement intervals at predetermined intervals is obviously undesirable. Photo-analysis techniques are complicated, not positive in detection and require that the patient be physically present in the physician's office. Further, such techniques are not readily available to physician and patient on short notice, but rather, as mentioned previously, would be available only at special clinics.
In U.S. Pat. No. 3,618,615, assigned to the assignee of this invention, there is disclosed an artificial cardiac pacemaker for generating at regular intervals a train of stimulating pulses, one of which is of significantly lower energy than the other pulses. If the heart responds to the reduced energy stimulating pulse, an adequate safety factor remains, but if the heart does not respond, e.g. no beat is detected in response to the lower energy or test pulse, marginal operation and possible imminent failure is ascertained.
Another method of ascertaining the pending failure of a pacemaker energy source is described in U.S. Pat. No. 3,713,449, assigned to the assignee of this invention, describing an artificial pacemaker including means for varying selectively the pulse width of its stimulating pulse. Control of the pulse width is made preferably by a mechanism external of the body by an attending physician. By such mechanism, the physician varies the pulse width of the implanted pacemaker until capture is lost. As the physician has previously measured the pulse width at the time of pacemaker implant, the pulse width at a subsequent time may be varied until capture is lost, whereby the state of the battery can be determined with respect to its replacement.
In an alternative approach to the problem of accurately determining battery depletion, there are artificial pacemakers such as described in U.S. Pat. No. 3,842,844 having a battery or cell depletion indicator that increases the pulse width of the output signal as their batteries deplete, i.e. their voltage amplitude decreases. Further, as the power source or battery depletes, the pulse repetition rate of such artificial cardiac pacemakers also decreases. For example, at the time of implantation, an artificial cardiac pacemaker may produce stimulating pulses at 70 beats per minute (BPM), plus or minus two beats, with a pulse width in the order of 0.5 msec. After a period of service illustratively in the order of 2-4 years, the BPM changes in the order of 5%-10%, i.e. a decrease of 5-7 beats from the original BPM, and the pulse width may increase to a value in the order of 1 msec. Dependent upon the known histories of such batteries, such a change in the BPM as well as a change in pulse width indicates that one of a plurality (e.g. 4 or 5) cells has failed, and that it is time to replace the batteries within the implanted pacemaker to assure continued heart stimulation of a sufficient level.
Pulse width increase is desired in order that as the amplitude of the voltage provided from the pacemaker battery decreases, the total energy in the stimulating pulse remains substantially constant. It is understood that the voltage level of the pacemaker battery may decrease below a lever at which the heart may not respond regardless of the pulse width. Further, as the pulse width increases to compensate for decreases in the voltage level, the current drain upon the battery increases, thereby increasing the rate of battery depletion.
As indicated above, one of the disadvantages of the prior art schemes with regard to determining pacemaker energy source depletion, is that it requires the patient to be physically present in the doctor's office, hospital or clinic. At the present time, there are systems such as described in U.S. Pat. No. 3,923,041, assigned to the assignee of this invention for sensing via electrodes attached to the patient's body the electrical activity of the heart including the stimulating pulses applied thereto from an artificial cardiac pacemaker and for transmitting such signals over readily available transmission media such as the telephone lines, to a remote station in the doctor's office, hospital or clinic, for example, where a trained person, such as the physician or his aide, may conveniently monitor the heart activity. In this manner, regular checkups of the patient's heart may be made without requiring the patient to travel to the hospital, clinic or doctor's office.
In FIG. 4 of the drawings, there is shown a typical graph of the electrical activity of a patient's heart which is stimulated by pulses derived from an artificial cardiac pacemaker. The heart activity or electrocardiogram (EKG) of the patient is indicated in FIG. 4 by the letter "B", whereas the stimulating pulse is identified by the letter "A", whose pulse width is indicated with the letters "PW". Noting that the threshold for stimulation of the heart varies from patient to patient, the minimum pulse width of the stimulating pulse is in the order of 100.mu.sec and its minimum voltage amplitude is in the order of 0.5V. Typically, the amplitude of the stimulating pulse would be in the order of 6V.
In view of the narrow bandwidth of telephone transmission lines, low carrier frequencies in the order of 1500Hz are used to transmit such signals. As a result, it is not possible to transmit with accuracy, the width of the pacemaker stimulating pulse. Typically, the pulse width of an artificial cardiac pacemaker may vary from 0.15 msec to about 3 msec. A significant portion of a pulse of such narrow width is lost when superimposed upon a low carrier frequency. Thus, though it would be possible to transmit directly such a pulse to a remote station, the detected pulse width would not be reproduced with sufficient accuracy to indicate with the desired precision the state or life of the pacemaker energy source.